I had a hum issue. Headache! Took ages to problem solve the issue as you start to question the foundations of your studios electronics for major ground loop issues..... however, in this instance if was an easy fix in the end! My simple fix: in my case the screw in the middle of the casing just needed tightening. As soon as I turned the screw the hum disappeared.
So anyone with a hum, simply try tightening the screws on the chasis of the mic!